I would do exactly as you suggested, listing him as backup for bot C and PF. In my experience a 3 man rotation is the best way to deal with the frontcourt: I've been using it for many seasons and I think you can have the top production (one of your best bigs will always be on the court instead of two backups) and minutes (always about 36 per starter, which I like). It's probably important that your lone bench player isn't a complete liability in JS and OD though, as he'll have to hold his own against forwards and not only centers.
That is, obviously, if you don't have training purposes for centers and power forwards. In that case you would be best suited to have two different young backups, in order to train more people.
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